According to datamined... data... in Wowhead, the reason our artifact weapons will not be carried over to the next expansion is that we will use them to suck up evil Sargeras' evil sword's evil power. Sluuuuuuuurp.

Quoth Magni Bronzebeard:
The sword of Sargeras is poisonin' the very lifeblood of the world.
The only vessels capable o' containin' that kind o' power...
are the mighty weapons you wield.
Raise them up as one.... draw out the corruption from the Dark Titan's blade!
Together, champions! For Azeroth!

What kind of sick individual burns a book full of perfectly good dark arts?!- Darkscryer Raastok